1. Field of the Invention
The invention relates to a process for waste water purification employing the activated sludge process with a ventilated activation tank from which treated waste water is continuously discharged.
2. Description of the Prior Art
It is a customary and generally-recognized rule of the art that the continuous, but usually not constant, flow rate of the waste water which is introduced into the biological stage of a waste water treatment plant is not manipulated. The result is a continuous charging of the aeration tanks, and within limits which are set, for example, on top by the separation duct and/or rainwater overflow, the flow rate of the waste water into the aeration tanks is a direct function of the inflow rate. In the case of domestic or urban waste water, this results in the known daily cycle of flow rate and pollution load expressed as COD (=chemical oxygen demand) or BOD.sub.5 (=biochemical oxygen demand after 5 days) concentration.
These daily cycles (or weekly cycles) of waste water load are generally reflected in fluctuations of treatment efficiency, that is, they lead to fluctuations of the COD and BOD.sub.5 concentration in the effluent, which is also clearly reflected in the permissible COD discharge values in accordance with waste water discharge laws. Accordingly, for example, the permissible COD maximum is twice the standard value (=operational average).
This shows that, so far, it has clearly not been possible to achieve constant purification performance within relatively narrow limits. Therefore, it seems desirable to develop a process which achieves a uniform discharge and guarantees a more effective protection of the water resources.
An additional problem in waste water treatment technology employing the activated sludge process is the uncontrolled formation of bulking sludge which is difficult to eliminate, especially with unfavorable C:N:P ratios (ratio of Carbon:Nitrogen:Phosphor source).
Even in the very earliest publications concerning the activated sludge process, it is considered advantageous to introduce the waste water alternately into two or more aeration tanks, so that there is a batch-wise operation for each individual tank (J. Soc. Chem. Ind. 33 (1914) 112). This method of operation has recently been propagated once again as the sequential batch reactor process (J. Water Pollution Control Fed. 51 (1980) 2747). A variant of the process is the continuously-fed activation tank with intermittent aeration and emptying (Publ. Health Eng. 8 (1980) 20; J. Water Pollution Control Fed. 56 (1984) 1160). Particular advantages of the last-named process are good eIimination rates when the waste water concentration and quantities fluctuate widely, and high dentrification rates.
According to another aerobic waste water purification method, such as in Austrian Pat. No. 32 18 33, the waste water is treated in batches under increased air pressure in activated sludge tanks with a sudden pressure release after the completion of the biological treatment, by means of which a certain sludge densification is achieved.
In laboratory tests with mixed pure bacterial cultures, Van den Eynde, et al. (European J. Appl. Microbiol. Biotechnol. 15 (1982) 246) achieved an elimination of the filamentous bulking sludge bacteria by the desired flocculants by means of intermittent substrate supply. However, the transition from laboratory findings with pure cultures to industrial waste water purification is not without its difficulties.